An optical fiber conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the cladding ng (i.e., nc>ng).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's cladding is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “alpha” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single mode fibers. In a multimode fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single mode fiber, the higher order modes are strongly attenuated. The typical diameter of a single mode or multimode optical fiber is 125 microns. The core of a multimode fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode fiber typically has a diameter of between about 6 microns and 9 microns. Multimode systems are generally less expensive than single mode systems because multimode light sources, connectors, and maintenance can be obtained at a lower cost.
Multimode fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network). Multimode fibers have been the subject of international standardization under the ITU-T G.651.1 standard, which, in particular, defines criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard is hereby incorporated by reference in its entirety.
In addition, the OM3 standard has been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE) over long distances (i.e., distances greater than 300 m). The OM3 standard is hereby incorporated by reference in its entirety. With the development of high-bandwidth applications, the average core diameter for multimode fibers has been reduced from 62.5 microns to 50 microns.
Typically, an optical fiber must have the broadest possible bandwidth for it to be usable in a high-bandwidth application. For a given wavelength, the bandwidth of an optical fiber may be characterized in several different ways. Typically, a distinction is made between the so-called “overfilled launch” condition (OFL) bandwidth and the so-called “Effective Modal Bandwidth” condition (EMB). The acquisition of the OFL bandwidth assumes the use of a light source exhibiting uniform excitation over the entire radial surface of the optical fiber (e.g., using a laser diode or light emitting diode (LED)).
Recently developed light sources used in high-bandwidth applications, such as VCSELs (Vertical Cavity Surface Emitting Lasers), exhibit an inhomogeneous excitation over the radial surface of the optical fiber. For this kind of light source, the OFL bandwidth is a less suitable measurement, and so it is preferable to use the effective modal bandwidth (EMB). The calculated effective bandwidth (EMBc) estimates the minimum EMB of a multimode fiber independent of the kind of VCSEL used. The EMBc is obtained from a dispersion-mode-delay (DMD) measurement.
FIG. 1 shows a schematic diagram of a DMD measurement according to the criteria of the FOTP-220 standard as published in its TIA SCFO-6.6 version of Nov. 22, 2002. The FOTP-220 standard is hereby incorporated by reference in its entirety. FIG. 1 shows a schematic representation of a part of an optical fiber (i.e., an optical core surrounded by an optical cladding). A DMD graph is obtained by successively injecting into the multimode fiber a light pulse having a given wavelength λ0 with a radial offset between each successive pulse. The delay of each pulse is then measured after a given length of fiber L. Multiple identical light pulses (i.e., light pulses having the same amplitude, wavelength, and frequency) are injected with different radial offsets with respect to the center of the multimode optical fiber's core. The injected light pulse is depicted in FIG. 1 as a black dot on the optical core of the optical fiber. In order to characterize an optical fiber with a 50-micron diameter, the FOTP-220 standard requires 24 individual measurements to be carried out (i.e., at 24 different radial offset values). From these measurements, it is possible to determine the modal dispersion and the calculated effective modal bandwidth (EMBc).
The TIA-492AAAC-A standard, which is hereby incorporated by reference in its entirety, specifies the performance requirements for 50-micron-diameter multimode optical fibers used over long distances in Ethernet high-bandwidth transmission network applications. The OM3 standard requires, at a wavelength of 850 nanometers, an EMB of at least 2000 MHz·km. The OM3 standard assures error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. The OM4 standard requires, at a wavelength of 850 nanometers, an EMB of at least 4700 MHz·km to obtain error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 550 meters. The OM4 standard is hereby incorporated by reference in its entirety.
In a multimode fiber, the difference between the propagation times, or group delay times, of the several modes along the optical fiber determine the bandwidth of the optical fiber. In particular, for the same propagation medium (i.e., in a step-index-type multimode fiber), the different modes have different group delay times. This difference in group delay times results in a time lag between the pulses propagating along different radial offsets of the optical fiber.
For example, as shown in the graph on the right side of FIG. 1, a time lag is observed between the individual pulses. The graph in FIG. 1 depicts each individual pulse in accordance with its radial offset in microns (y-axis) and the time in nanoseconds (x-axis) the pulse took to pass along a given length of the optical fiber.
As depicted in FIG. 1, the location of the peaks along the x-axis varies, which indicates a time lag (i.e., a delay) between the individual pulses. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse (i) increases the risk of the pulse being superimposed onto a following pulse and (ii) reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is directly linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode theory of graded-core fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive theory of dispersion in graded-index optical fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.
A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:
  n  =            n      1        ⁢                  1        -                  2          ⁢                                    Δ              ⁡                              (                                  r                  a                                )                                      α                              
wherein,
α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;
n1 is the maximum refractive index of the optical fiber's core;
a is the radius of the optical fiber's core; and
  Δ  =            (                        n          1          2                -                  n          0          2                    )              2      ⁢              n        1        2            
wherein n0 is the minimum index of the multimode core, which generally corresponds to the index of the cladding (most often made of silica).
A multimode fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices), which affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.
In practice, however, a manufactured multimode fiber has a graded-index central core surrounded by an outer cladding of constant index. Thus, the core of the multimode fiber never corresponds to a theoretically perfect alpha profile (i.e., the alpha set profile), because the interface of the core (having an alpha profile) with the outer cladding (having a constant index) interrupts the alpha profile. The outer optical cladding accelerates the higher-order modes with respect to the lower-order modes. This phenomenon, known as the “cladding effect,” can be seen in the graph of the DMD measurements of a simulated fiber presented in FIG. 2.
As shown in FIG. 2, the response signals acquired for the highest radial positions (i.e., a high radial offset, such as above 20 microns) exhibit multiple pulses. These “dual-pulses” are attributable to the fact that the higher-order modes are accelerated when propagating through the cladding rather than through the core. In this regard, these accelerated higher-order modes will arrive at different times than will the lower-order modes. The presence of such multiple pulses is reflected in a temporal broadening of the resulting response signal. As a result of this cladding effect, the bandwidth is reduced. Therefore, to achieve the performance requirements of the TIA-492AAAC-A standard, the cladding effect in the optical fiber should be reduced, if not eliminated.
International Publication No. WO 2006/010798 and its counterpart U.S. Publications Nos. 2009/0052851 and 2010/0098431, each of which is hereby incorporated by reference in its entirety, describe a multimode optical fiber that includes a graded-index central core and a depressed trench located at the periphery of the central core. The graded-index profile of the central core is extended to the bottom of a depressed trench (i.e., an extended depressed gradient core), which is followed by a depression of constant refractive index. This kind of prior art profile is shown in FIG. 10C, which is further explained below. The extended depressed gradient core is considered to be part of the core and the depression of constant refractive index is considered to be a depressed trench. The extension of the alpha-index core under the outer optical cladding and to the bottom of the depressed trench (i.e., an extended depressed gradient core) can lead to an increase in the size of the core. Increasing the size of the core, however, can result in incompatibility with the aforementioned OM3 and OM4 standards. The extension of the core to negative refractive index values can also generate losses because of the propagation of leaky modes, which are intrinsic to the geometry of a depressed trench.
Commonly assigned International Publication No. WO 2009/054715, which is hereby incorporated by reference in its entirety, discloses a multimode optical fiber that includes a central core having a graded-index profile and a depressed cladding positioned at the periphery of the central core. This graded-index profile of the core is extended to the bottom of the depressed trench and is followed by a depression of constant refractive index. As previously discussed, this kind of refractive index profile is shown in FIG. 10C, which is further explained below. Furthermore, this kind of prior art profile can lead to (i) an increase in the size of the core, (ii) incompatibility with the OM3 and OM4 standards, and/or (iii) losses caused by the propagation of leaky modes.
U.S. Pat. No. 4,339,174, which is hereby incorporated by reference in its entirety, describes a multimode fiber including a core having a graded-index profile and having a reduced cladding effect. The optical fiber, however, has a diameter of 60.5 microns and is not particularly suitable for high-bandwidth applications.
U.S. Pat. No. 4,184,744, which is hereby incorporated by reference in its entirety, discloses a multimode optical fiber that includes a central core having a graded-index profile and a depressed trench (called intermediate layer) situated at the periphery of the central core. The thickness of the depressed trench is between 0.1 and 1 times the radius of the core. Thus, for a core having a radius of 50 microns, the thickness of the intermediate layer (i.e., the depressed trench) is between 5 and 50 microns. This thick intermediate layer eliminates the highest order modes (i.e., the modes that are influenced the most by the cladding effect) but undesirably reduces the bandwidth of the optical fiber.
U.S. Pat. Nos. 4,229,070 and 4,230,396, each of which is hereby incorporated by reference in its entirety, describe multimode fibers having a graded-index core surrounded by a depressed trench in order to reduce the cladding effect. The optical fibers, however, have a diameter of 62.5 microns and are not suitable for high-bandwidth applications.
Therefore, a need exists for a high-bandwidth multimode fiber (i) that has a refractive index profile including a graded-index core and a depressed trench and (ii) that reduces the cladding effect.